Over the past several decades the U.S. has achieved remarkable success in reducing air pollution. According to the Environmental Protection Agency, the country has reduced six common air pollutants by 72% since 1970. These reductions are credited with achieving meaningful public-health benefits, from improved respiratory health to increased life expectancy.

Yet with this success we now face a critical question: Will further decreases in air pollution to levels that approach those that occur naturally necessarily result in additional public-health benefits? This question gets to the heart of the EPA's current evaluation of whether the existing National Ambient Air Quality Standard for ozone is sufficient to protect public health. Ozone is a colorless, odorless gas that is not directly emitted into air, but is formed when sunlight reacts with two other pollutants: volatile organic compounds and nitrogen oxides. These come from many natural sources (plants, forest fires) as well as human-made sources (cars, industrial facilities, power plants).

The Clean Air Scientific Advisory Committee, a panel of scientists and public-health experts charged by Congress with advising the EPA, met in March to discuss the agency's evaluation of the link between ozone and respiratory illnesses such as asthma and other health issues. The hope is that, with robust public input, the EPA and the committee will arrive at conclusions that accurately reflect the current state of scientific research on ozone. The stakes are significant: The EPA itself estimates that more-stringent standards could cost businesses up to $90 billion annually.

Currently the EPA standard for ozone in the air is 75 parts per billion, the strictest level since the standard was established in 1971. In 2008 the EPA determined, and a federal court agreed, that this standard protects public health. But now the EPA says that 75 ppb is not protective enough and is recommending a change to between 60 ppb and 70 ppb. Meanwhile, the overwhelming body of scientific evidence indicates lowering the current ozone standard will not provide added health benefits beyond those achieved with the current standard.

There have been hundreds of scientific studies on ozone exposure and possible health effects, and the EPA has reviewed most of them. However, the EPA has not evaluated them in systematic fashion, by considering study strengths and limitations in a consistent manner from study to study. This type of analysis is called a "weight-of-evidence" evaluation, and it can help prevent the cherry-picking of studies—which can occur when scientists focus on studies or evaluate data that confirm their position, or when the scientists place less emphasis on studies that do not.

Most studies examining connections between ozone and health effects do not adequately account for smoking or other factors such as diet and exercise that could contribute to diseases attributed to ozone. By not fully considering these other factors, the EPA assumes that ozone causes more health effects than what the science supports.

The EPA also has interpreted some studies to indicate that ozone is more harmful than it likely is. For example, Dr. William C. Adams, professor emeritus at the University of California, Davis, published a peer-reviewed paper in 2006 concluding that 60 ppb ozone exposures to people exercising for six hours did not lead to harmful respiratory effects. The EPA "reanalyzed" his data and determined that it did. Dr. Adams later said, on the EPA Docket for public comment, that the "EPA has misinterpreted the statistics contained in my published, peer-reviewed paper."

This kind of discrepancy should give the Clean Air Scientific Advisory Committee pause as it evaluates the health effects of ozone.

The EPA considers worst-case scenarios arguably to protect the most sensitive people in a population. However, in its ozone health-risk and exposure assessment, the EPA makes many "worst-case" assumptions that could not all occur at one time, leading to an unrealistic scenario that overestimates risks.

The lowered standard that the EPA is proposing (between 60 and 70 ppb) is very close to levels of ozone that are found naturally in some regions of the country. For example, the ozone level at Big Bend National Park in southwest Texas, where there are no industrial facilities, is 71 ppb.

Significantly, the EPA's models have shown that decreasing human-made sources of ozone could actually lead to increased natural ozone in some areas. This happens because ozone formation is complex, and nitrogen oxides both form and destroy ozone. By controlling human sources of nitrogen dioxide to achieve lower ozone levels, many parts of the country will not be able to meet current ozone standards solely because of naturally formed ozone.

We all want appropriate standards to ensure that our air is clean. But the EPA shouldn't lower the ozone standard unless there is solid evidence that doing so would result in measurable improvements in health or reductions in the diseases associated with air pollution.

Drs. Goodman and Sax consult for business and government on issues relating to the EPA's air-pollution standards. Dr. Goodman also is an adjunct faculty member of the Harvard School of Public Health.